Balloon telemetry data using radio teletype or RTTY (Baudot)
My project is to send aloft, a balloon carrying a GPS receiver, 2M FM amateur radio and a digital camera. Wanting to 'roll my own' or 'homebrew' the payload TM circuitry rather than buying it, steered me into using RTTY.
While APRS and AX.25 provide data integrity, my programming skills lacked the expertise to implement this method. So I started thinking about using the amateur radio standard AFSK RTTY at 170Hz shift. Data integrity should be maintained fairly accurate because the receiving station and payload will be in Line of Site with each other. The payload TM signal should be very strong at the receiving station(s).
Searching this site and using Google, I could only find a couple of articles in the PICAXE forum and none on this forum. My PIC of choice is the PIC16F627.
Reading the GPS NEMA 0183 data was simple, converting the $GPGGA NEMA string ASCII data into Baudot and readying it for transmit it was a bit trickier.
I have a test array sending Baudot to my o'scope and the data pulses are 22mS, and meet the 45.5 baud Baudot format.
I welcome any comments, suggestions or critiques.
Here is my code:
Code:'PICBASIC PRO program to read a 4800 BAUD GPS GPGGA NEMA ' 0183 string and transmit it using BAUDOT (RTTY) with standard amateur ' radio AFSK 170Hz shift, using the XR2206 modem. ' ' INCLUDE "BS2DEFS.BAS" DEFINE HSER_CLROERR 1 'USART CLEAR OVERFLOW ERROR DEFINE HSER_TXSTA 24H 'SET USART TX STAUS REG DEFINE HSER_RCSTA 90H 'SET USART RC STAUS REG DEFINE HSER_BAUD 4800 'SET USART BAUD RATE define HSER_SPBRG 50 'SET PIC OSC FOR 4800 BAUD RXDATA var BYTE (80) 'ALLOCATE 80 BYTES FOR USART RX DATA PIR1 = $0C 'ASSIGN USART RX INTERUPT REG PTT VAR PORTB.2 'ASSIGN PTT PIN O Var PORTB.1 'ASSIGN BAUDOT OUTPUT PIN I VAR BYTE L VAR BYTE '---------------------READ $GPGGA DATA----------------------------- ' Read serial port and wait for "GGA", then load RXDATA with entire ' GPGGA string. Continue loading data for the next 80 characters or ' unitl a "$" is detected. ' Sample GPGGA string: ' $GPGGA,150735.000,3444.9047,N,11849.3610,W,1,09,0.9,1019.8,M, ' -31.9,M,,0000*50 LOADDATA: HSERIN [WAIT (71,71,65), STR RXDATA \80\24] '---------------------PTT------------------------------------------ ' Send PTT to radio and wait .5 seconds. Your radio interface circuitry ' should receive this PTT signal. HIGH PTT PAUSE 500 '---------------------MARK Tone------------------------------------ ' Transmit Mark tone of 2295Hz for 2 seconds. No reason, I just ' want to... HIGH O PAUSE 2000 '---------------------SEND LTRS 5X TO SYNC RCV STATION------------- ' Transmit LTRS data "11111" 5 times to synchronize recieving station's ' timing. This alerts the receiving station that the next characters are ' BAUDOT letter characters. The LTRS data only needs to be sent one time. FOR I = 1 TO 5 GOSUB RTTYLTR NEXT I '----------------------SEND FIGS 5X-------------------------------- ' Transmit FIGS data "11011" 5 times. This alerts recieving station ' that the next characters are to be BAUDOT FIG characters. The FIGS ' data only needs to be sent one time. FOR I = 1 TO 5 GOSUB RTTYFIG NEXT I '----------------------SEND CR AND LF ----------------------------- ' Transmit Carriage Return (CR) and Line Feed (LF) 5 times to ' format receiving station display. This ensures the received GPS ' Baudot data will start on a new line. FOR I = 1 TO 5 GOSUB RTTYCRLF NEXT I '----------------------Read GPS ASCII Data------------------------- ' Read RXDATA and convert it to BAUDOT. Since we are only expecting ' 14 unique characters in the NEMA GPGGA string, there is no need to ' convert the entire alphabet and other special characters to BAUDOT. ' Depending on your geographic location, you may want to add an "E" ' and/or "S" to the RXDATA and character routines below. FOR L = 0 TO 80 IF RXDATA (L) = 0 THEN GOSUB RTTY0 IF RXDATA (L) = 1 THEN GOSUB RTTY1 IF RXDATA (L) = 2 THEN GOSUB RTTY2 IF RXDATA (L) = 3 THEN GOSUB RTTY3 IF RXDATA (L) = 4 THEN GOSUB RTTY4 IF RXDATA (L) = 5 THEN GOSUB RTTY5 IF RXDATA (L) = 5 THEN GOSUB RTTY6 IF RXDATA (L) = 7 THEN GOSUB RTTY7 IF RXDATA (L) = 8 THEN GOSUB RTTY8 IF RXDATA (L) = 9 THEN GOSUB RTTY9 IF RXDATA (L) = "N" THEN GOSUB RTTYN IF RXDATA (L) = "W" THEN GOSUB RTTYW IF RXDATA (L) = "," THEN GOSUB RTTYSPACE 'Convert "," to SPACE IF RXDATA (L) = "." THEN GOSUB RTTYPERIOD NEXT L '---------------------DISABLE PTT AFTER ALL DATA IS TRANSMITTED---- LOW PTT '------------------------------------------------------------------ PAUSE 60000 'WAIT 60 SECONDS GOTO LOADDATA 'GO BACK AND READ GPS DATA AGAIN '---------------------BAUDOT BIT TX SUBROUTINES-------------------- ' EACH BAUDOT WORD CONTAINS A 22mS LOW START BIT, 5 22mS HIGH OR LOW ' DATA BITS AND A 44mS HIGH STOP BIT. THE BAUDOT DATA IS SENT LSB FIRST. ' THE ENTIRE BAUDOT WORD IS 176mS LONG. ' START BIT DATA BITS STOP BIT ' 22mS + 110mS + 44mS ' ' IN THE RTTY0: SUBROUTINE BELOW, L2 BRINGS PORTB.1 LOW FOR 44mS(22mS X 2). ' THE FIRST 22mS LOW IS THE START BIT AND THE LAST 22mS IS BIT0. H2 ' BRINGS PORTB.1 HIGH FOR 44mS. THE FIRST 22mS IS BIT1 AND THE LAST 22mS ' IS FOR BIT2...ETC. ' ' L1 = LOW PORTB.1 FOR 22mS ' H2 = HIGH PORTB.1 FOR 44mS ' H5 = HIGH PORTB.1 FOR 110mS...ETC ' RTTY0: 'TX BAUDOT "0" GOSUB L2 'TX START BIT and BIT0(LSB) GOSUB H2 'TX BIT1 and BIT2 GOSUB L1 'TX BIT3 GOSUB H3 'TX BIT4(MSB) and STOP Bit RETURN RTTY1: 'TX BAUDOT "1" GOSUB L1 'TX START BIT GOSUB H3 'TX BIT0, BIT1 AND BIT2 GOSUB L1 'TX BIT4 GOSUB H3 'TX BIT4(MSB) and STOP Bit RETURN RTTY2: 'TX BAUDOT "2" GOSUB L1 'TX START BIT GOSUB H2 'TX BIT0 AND BIT1 GOSUB L2 'TX BIT3 GOSUB H3 'TX BIT4(MSB) and STOP Bit RETURN RTTY3: 'TX BAUDOT "3" OR 01100011 GOSUB L1 'TX START BIT GOSUB H2 'TX BIT0 AND BIT1 GOSUB L3 'TX BIT2, BIT3 AND BIT4 GOSUB H2 'TX STOP BIT RETURN RTTY4: 'TX BAUDOT "4" GOSUB L2 'TX START BIT and BIT0(LSB) GOSUB H1 'TX BIT1 GOSUB L1 'TX BIT2 GOSUB H1 'TX BIT3 GOSUB L1 'TX BIT4 GOSUB H2 'TX STOP BIT RETURN RTTY5: 'TX BAUDOT "5" GOSUB L5 'TX START BIT AND BIT0 - BIT3 GOSUB H3 'TX BIT4 and STOP Bit RETURN RTTY6: 'TX BAUDOT "6" GOSUB L1 'TX START BIT GOSUB H1 'TX BIT 0 GOSUB L1 'TX BIT1 GOSUB H1 'TX BIT2 GOSUB L1 'TX BIT3 GOSUB H3 'TX BIT4(MSB) and STOP Bit RETURN RTTY7: 'TX BAUDOT "7" GOSUB L1 'TX START BIT GOSUB H3 'TX BIT0 - BIT2 GOSUB L2 'TX BIT3 AND BIT4 GOSUB H2 'TX STOP BIT RETURN RTTY8: 'TX BAUDOT "8" GOSUB L2 GOSUB H2 GOSUB L2 GOSUB H2 RETURN RTTY9: 'TX BAUDOT "9" GOSUB L4 GOSUB H4 RETURN RTTYN: 'TX BAUDOT "N" GOSUB RTTYLTR GOSUB L3 GOSUB H2 GOSUB L1 GOSUB H2 GOSUB RTTYFIG RETURN RTTYW: 'TX BAUDOT "W" GOSUB RTTYLTR GOSUB L1 'TX START BIT GOSUB H2 GOSUB L2 GOSUB H3 'TX BIT4(MSB) and STOP Bit GOSUB RTTYFIG RETURN RTTYSPACE: 'TX BAUDOT "SPACE" GOSUB L3 GOSUB H1 GOSUB L2 GOSUB H2 RETURN RTTYPERIOD: 'TX BAUDOT "." GOSUB L3 GOSUB H5 RETURN RTTYCRLF: 'TX BAUDOT CR & LF GOSUB L4 GOSUB H1 GOSUB L1 GOSUB H2 GOSUB L2 GOSUB H1 GOSUB L3 GOSUB H2 RETURN RTTYLTR: 'TX BAUDOT LTR GOSUB L1 'TX START BIT GOSUB H7 'TX SEVEN HIGH PULSES BIT0 - BIT4 AND A STOP BIT RETURN RTTYFIG: 'TX BAUDOT FIG GOSUB L1 'TX START BIT GOSUB H2 GOSUB L1 GOSUB H4 RETURN '-------------------RTTY HIGH/LOW PULSE LENGTH SUBS---------------- ' IN BAUDOT, THE START BIT IS LOW FOR 22mS AND EACH DATA BIT IS 22mS ' IN DURATION AND EITHER A LOW (0) OR HIGH (1). WHILE THE STOP BIT ' IS HIGH FOR 44mS. L1: 'HOLD PORTB.1 LOW FOR 22mS IN DURATION LOW O PAUSE 22 RETURN L2: 'HOLD PORTB.1 LOW FOR 44mS IN DURATION LOW O PAUSE 44 RETURN L3: 'HOLD PORTB.1 LOW FOR 66mS IN DURATION LOW O PAUSE 66 RETURN L4: 'ETC LOW O PAUSE 88 RETURN L5: 'ETC LOW O PAUSE 110 RETURN H1: 'HOLD PORTB.1 HIGH FOR 22mS IN DURATION HIGH O PAUSE 22 RETURN H2: 'HOLD PORTB.1 HIGH FOR 44mS IN DURATION HIGH O PAUSE 44 RETURN H3: 'ETC HIGH O PAUSE 66 RETURN H4: 'ETC HIGH O PAUSE 88 RETURN H5: 'ETC HIGH O PAUSE 110 RETURN H7: 'ETC HIGH O PAUSE 154 RETURN End
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